High fidelity amplifiers maintain good linear characteristics for source signal having normal amplitudes.
However, for excess source levels, the amplfiers go beyond their linear range and are saturated. This is called "clipping" because the amplifiers output clipped or truncated signals.
For audio-applications, clipping distortions impair sound quality and clarity and sometimes damage a loud-speaker coupled to the output of the amplifier.
U.S. Pat. No. 4,048,573 to Evans et al discloses an apparatus which reduces clipping distortions. FIG. 1 is originally described in this United States Patent. The source signal passes through a preamplifier 10 to a main or power amplifier 20. The output signal of the power amplifier 20 is provided to one input of a differential amplifier 30 through a feedback circuit 25. The output signal of the preamplifier is also provided to the other input of the differential amplifier 30. The function of the differential amplifier is to produce a signal indicative of the occurence of clipping when the input and output signals of the power amplifiers differ. The output signal from the differential amplifier is fed to a full wave rectifier 32, the output of which drives an attenuator 31 as well as a limit lamp driver 33.
In normal operation (i.e. when the input signal has normal amplitudes), the differential amplifier does not develop any signal because its input signals are the same in strength and waveform. Thus, no attenuation is introduced in the signal applied to the power amplifier.
On the other hand, when an excess input is fed to the power amplifier 20, the latter will produce clipping. Therefore, the differential amplifier 30 produces an output signal in accordance with the difference between the amplifier input signal and the amplifier output signal through the feedback circuit 25. This difference will introduce a corresponding attenuation in the input signal to the amplifier 20 by means of full wave rectifier 32 and signal attenuator 31 coupled to the output of the differential amplifier 30.
While the apparatus disclosed in this United States Patent serves to minimize clipping distortions, the control of signal attenuation does not start until the clipping occurs and is observed at the differential amplifier 30. This is a "post clipping" control method. In other words, the technique utilized in this United States Patent is to compare amplifier input with output signals. Since the amplifier output signal is also distorted depending upon the characteristics of the load (e.g. inductive or capacitive loud-speaker), this latter distortion, i.e. the distortion caused by factors other than clipping, would be detected by the differential amplifier 30 if the latter has a very high sensitivity, with the result that the amplifier input signal is attenuated. Such erroneous attenuation in the input signal is undesirable for the purpose of maintaining a high power output signal at the amplifier output.